The second tube
will measure the pressure at that point. If the pressure there was equal to the pressure in the res (which you're implying is the case by saying that the tube will read the same level as the reservoir) then there's no driving force behind the flow from that point on. Flow occurs because of a pressure differential (well, energy really, but whatever...). Really, honest... I've used a differential manometer before to measure the permeability of concretes and soils, so I am sure about this. But hey, if you don't believe me, ask Dave or BillA.